Vertebrate taste transduction is mediated by specialized neuroepithelial cells, referred to as taste receptor cells. Groups of forty to one hundred taste receptor cells form a taste bud, the vast majority of which are embedded within the epithelium of the tongue. Each taste bud has a taste pore through which the taste receptor cells are exposed to the chemical environment in the mouth. Various taste stimulants (tastants) come into contact with the taste receptor cells and cause the taste receptor cells to send a signal to the brain by releasing neurotransmitter(s). Afferent nerve fibers that enter each taste bud receive the neurotransmitter signal.
There are four basic taste modalities typified by four distinct groups of tastants: salty, sour, sweet, and bitter. Different taste modalities appear to involve different signalling pathways and mechanisms. For example, salty taste appears to be mediated by sodium ion flux through apical sodium channels and sour taste seems to be mediated via hydrogen ion blockade of potassium or sodium channels.
Of particular interest to the background of the present invention are guanine nucleotide binding proteins (G proteins) which have been specifically implicated in the transduction of sweet and bitter tastes and may also be involved in the regulation of the ion channels involved in transduction of salty and sour tastes. Briefly, G proteins are heterotrimeric proteins (each having an α, β, and γ subunit) which mediate signal transduction in olfactory, visual, hormonal and neurotransmitter systems. G proteins couple cell surface receptors to cellular effector enzymes (e.g., phosphodiesterases and adenylate cyclase) or ion channels and thereby transduce an extracellular signal into an intracellular second messenger (e.g., cAMP, cGMP, IP3). G protein α and βγ subunits separately and jointly regulate the activity of effector enzymes. Downstream events initiated by the effector enzymes result in release of neurotransmitter from the taste receptor cells. While the α subunit of a G protein is thought to confer most of the specificity of interaction between its receptor and its effectors, βγ dimers also contribute to the specificity of receptor coupling and to the regulation of receptor phosphorylation and desensitization. A number of G proteins are ubiquitously expressed (e.g., Gs and Gi), but others that are known to be involved in sensory transduction have been found only in specialized sensory cells. For example, Lerea et al., Science, 224: 77–80 (1986) reports that transducin (Gt) transduces photoexcitation in retinal rod and cone cells and Ruiz-Avila et al., Nature, 376: 80–85 (1995) describes its participation in transduction of bitter taste in taste receptor cells. Jones et al., Science, 244: 790–795 (1989) reports that Golf transduces olfactory stimulation in neurons of the olfactory epithelium. The ubiquitously expressed G proteins may also be involved in sensory transduction.
Experimental evidence that G proteins are involved in the taste transduction pathway is described in several publications, including publications authored by co-inventors herein. Wong et at., Cold Spring Harb. Symp. Quant. Biol., 61, 173–184 (1996) demonstrates the role of a G protein, gustducin, in bitter and sweet taste transduction. The article reports behavioral studies revealing that mice which do not express the gustducin α subunit exhibit insensitivity to various bitter and sweet compounds in comparison to normal mice. Moreover, the article reports that the mice exhibited significant descreases in nerve responses to the bitter and sweet compounds indicating that the bitter and sweet taste signalling pathway(s) had been interrupted by lack of expression of the gustducin α subunit. The gustducin α subunit has been described in terms of its DNA and amino acid sequences in U.S. Pat. No. 5,688,662 to Robert F. Margolskee. However, the β and γ subunits of the heterotrimeric gustducin G protein had not been previously identified or characterized.
Over the past two decades substantial efforts have been directed to the development of various agents that interact with taste receptors to mimic or block natural taste stimulants. Examples of agents that have been developed to mimic sweet tastes are saccharin (an anhydride of o-sulfimide benzoic acid) and monellin (a protein) and the thaumatins (also proteins). Thaumatins have been utilized as additives in food, cigarette tips, medicines and toothpaste. Many taste-mimicking or taste-blocking agents developed to date are not suitable as food additives, however, because either they are not economical or are high in calories, or because they are carcinogenic. Development of new agents that mimic or block the four basic taste modalities has been limited by a lack of knowledge of the taste receptor cell proteins responsible for transducing the taste modalities.
There thus continues to exist a need in the art for new products and methods that are involved in or affect taste transduction.